Conventionally, an electric power steering device (EPS) having a motor as a drive source is known as a power steering device for a vehicle. Compared to hydraulic power steering devices, the EPS characteristically enables flexible layout and reduces energy consumption. Accordingly, use of the EPS is now considered in not only small-sized vehicles but also large-sized vehicles. As a result, corresponding improvement is strongly demanded in the output performance of the EPS.
However, there is actually only a limited space for installing an EPS actuator in a vehicle. Particularly, if the EPS is a rack type or a pinion type, it is practically impossible to accommodate a large-sized motor. Further, in the case of a column type EPS, which occupies a relatively small space, the weight of the device is disadvantageously increased by reinforcing a steering shaft in such a manner as to meet increase of output.
To solve these problems, it is demanded to provide an EPS that increases output while avoiding problems related to limited installation space and weight increase, and ensures both redundancy and high reliability. In this regard, an EPS employing two motors has been proposed (see, for example, Patent Document 1). One of the motors applies assist force to a rack shaft and the other applies assist force to a steering shaft. Also, since EPS's are now used in a wider variety of vehicles, improved qualities are demanded in the devices. Particularly, an extremely high standard is demanded for quietness of operation. To meet this demand, for example, an EPS that carries out compensation control to inhibit influence of a torque ripple caused by erroneous detection of a sensor or an EPS that reduces cogging torque of a motor has been proposed (see, for example, Patent Documents 2 or 3).    Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-82798    Patent Document 2: Japanese Laid-Open Patent Publication No. 2005-186775    Patent Document 3: Japanese Laid-Open Patent Publication No. 2003-324867
Conventionally, in many cases, to meet demand for a small-sized and higher output EPS, the EPS employs, as a motor serving as a drive source, a permanent magnet motor having permanent magnets, such as a brushless motor (a BLDC motor), for a rotor. However, the permanent magnet motor has an essential problem, which is cogging torque. The influence of the cogging torque on steering feel becomes greater as the output of the EPS becomes greater. This problem cannot be solved even if a plurality of motors are mounted in the EPS, as long as the permanent magnet motor serves as the drive source. Also, if multiple motors are employed, it is important to match the control phases of the motors. However, in the EPS in which one of the motors operates to assist the rack shaft and the other operates to assist the steering shaft, it is difficult to match the control phases of the two motors due to torsion of the steering shaft. This may cause interference between control of one motor with control of the other, thus deteriorating the steering feel together with the togging torque.